The present invention relates to a composite catalyst incorporating on the one hand a MFI structure zeolite containing silicon and aluminium and on the other hand a metal from the platinum group deposited on a refractory oxide support and to which are added at least one metal additive chosen from the group constituted by tin, germanium, indium and lead and at least one alkali metal or alkaline earth metal.
The invention also relates to the use of this composite catalyst in the aromatization reactions of hydrocarbons having 2 to 4 carbon atoms per molecule.
The aromatization reaction of propane and butane in the presence of a catalyst containing the zeolite ZSM5 (or MFI) and gallium was discovered and patented by British Petroleum (U.S. Pat. No. 4,175,057 and U.S. Pat. No. 4,180,689). Since this date other organizations and companies have filed patent applications concerning modifications of the solid (U.S. Pat. No. 4,795,844 European Patent EP-B-252,705) and/or to changes of the charge: C.sub.2 -C.sub.12 fraction (European Patent EP-B-252,705), ethane and ethylene (European Patent EP-B-50,021 and U.S. Pat. No. 4,350,835). The method of introducing the gallium is also covered by patents (European Patents EP-B-120,018 and 184,927).
The addition of another metal to the system Ga-MFI has been envisaged for improving the aromatic selectivity and for reducing the coke quantity on the catalyst. Thus, the impregnation of a Ga/MFI catalyst by rhenium, associated with platinum or palladium makes it possible to significantly improve the aromatic production selectivity (U.S. Pat. No. 4,766,265). Other patents claim the addition of platinum and palladium to the Ga-MFI catalyst (U.S. Pat. No. 4,407,728, European Patents EP-B-215,579, 216,491, 224,162, 228,267 and Japanese Patents 61-268,634, 62-081,329, 61-2,277,636).
The addition of platinum to the MFI zeolite makes it possible to improve the propane conversion (T. INUI, F. OKAZUNI, Y. MAKINO, Chem. Express 1 (1), 53-56, 1985). However, the methane and ethane production selectivity is significantly improved. The addition of rhenium to the platinum further improves the production of methane and ethane by hydrogenolysis (S. ENGELS et al, Catalysis Today, 3, pp. 437-443, 1988). The addition of copper (P. MERIAUDEAU et al, Zeolites: Facts, Figures, Future, pp. 1423-1429, 1989) and chromium (E. S. SHPIRO et al, International Symposium on Zeolites as catalysts, Sorbents and Detergent builders, Wurzburg (GFR), p. 73, 1988) reduces methane production, but the aromatic selectivity remains inferior to Ga/MFI systems. Moreover, very recently the addition of sulphur to a Pt/MFI catalyst makes it possible to significantly improve its production selectivity to aromatics produced from paraffins containing 6 to 12 carbon atoms (U.S. Pat. No. 4,835,336).